doc: Add mention of the /config binding

The devicetree binding files are in their own directory and use a simple
text format. Add a link for the binding for the /config node, since it
is otherwise hard to find.

Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>